# std::experimental::is_detected, std::experimental::detected_t, std::experimental::detected_or

From cppreference.com
< cpp‎ | experimental

Defined in header `<experimental/type_traits>` |  |
template< template<class...> class Op, class... Args > using is_detected = /* see below */; |  | (library fundamentals TS v2)
template< template<class...> class Op, class... Args > using detected_t = /* see below */; |  | (library fundamentals TS v2)
template< class Default, template<class...> class Op, class... Args > using detected_or = /* see below */; |  | (library fundamentals TS v2)

The alias template `detected_or` is an alias for an unspecified class type with two public member typedefs `value_t` and `type`, which are defined as follows:

- If the template-id Op<Args...> denotes a valid type, then `value_t` is an alias for std::true_type, and `type` is an alias for Op<Args...>;
- Otherwise, `value_t` is an alias for std::false_type and `type` is an alias for `Default`.

The alias template `is_detected` is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::value_t. It is an alias for std::true_type if the template-id Op<Args...> denotes a valid type; otherwise it is an alias for std::false_type.

The alias template `detected_t` is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::type. It is an alias for Op<Args...> if that template-id denotes a valid type; otherwise it is an alias for the class `std::experimental::nonesuch`.

### Additional utilities

template< template<class...> class Op, class... Args > constexpr bool is_detected_v = is_detected<Op, Args...>::value; |  | (library fundamentals TS v2)
template< template<class...> class Op, class... Args > constexpr inline bool is_detected_v = is_detected<Op, Args...>::value; |  | (library fundamentals TS v3)
template< class Default, template<class...> class Op, class... Args > using detected_or_t = typename detected_or<Default, Op, Args...>::type; |  | (library fundamentals TS v2)
template< class Expected, template<class...> class Op, class... Args > using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>; |  | (library fundamentals TS v2)
template< class Expected, template<class...> class Op, class... Args > constexpr bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value; |  | (library fundamentals TS v2)
template< class Expected, template<class...> class Op, class... Args > constexpr inline bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value; |  | (library fundamentals TS v3)
template< class To, template<class...> class Op, class... Args > using is_detected_convertible = std::is_convertible<detected_t<Op, Args...>, To>; |  | (library fundamentals TS v2)
template< class To, template<class...> class Op, class... Args > constexpr bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value; |  | (library fundamentals TS v2)
template< class To, template<class...> class Op, class... Args > constexpr inline bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value; |  | (library fundamentals TS v3)

The alias template `is_detected_exact` checks whether detected_t<Op, Args...> is `Expected`.

The alias template `is_detected_convertible` checks whether detected_t<Op, Args...> is convertible to `To`.

### Possible implementation

    namespace detail
    {
        template<class Default, class AlwaysVoid, template<class...> class Op, class... Args>
        struct detector
        {
            using value_t = std::false_type;
            using type = Default;
        };

        template<class Default, template<class...> class Op, class... Args>
        struct detector<Default, std::void_t<Op<Args...>>, Op, Args...>
        {
            using value_t = std::true_type;
            using type = Op<Args...>;
        };
    } // namespace detail

    template<template<class...> class Op, class... Args>
    using is_detected = typename detail::detector<nonesuch, void, Op, Args...>::value_t;

    template<template<class...> class Op, class... Args>
    using detected_t = typename detail::detector<nonesuch, void, Op, Args...>::type;

    template<class Default, template<class...> class Op, class... Args>
    using detected_or = detail::detector<Default, void, Op, Args...>;

### Example

Run this code

    #include <cstddef>
    #include <experimental/type_traits>

    template<class T>
    using copy_assign_t = decltype(std::declval<T&>() = std::declval<const T&>());

    struct Meow {};
    struct Purr { void operator=(const Purr&) = delete; };

    static_assert(std::experimental::is_detected<copy_assign_t, Meow>::value,
                  "Meow should be copy assignable!");
    static_assert(!std::experimental::is_detected_v<copy_assign_t, Purr>,
                  "Purr should not be copy assignable!");
    static_assert(std::experimental::is_detected_exact_v<Meow&, copy_assign_t, Meow>,
                  "Copy assignment of Meow should return Meow&!");

    template<class T>
    using diff_t = typename T::difference_type;

    template<class Ptr>
    using difference_type = std::experimental::detected_or_t<std::ptrdiff_t, diff_t, Ptr>;

    struct Woof { using difference_type = int; };
    struct Bark {};

    static_assert(std::is_same<difference_type<Woof>, int>::value,
                  "Woof's difference_type should be int!");
    static_assert(std::is_same<difference_type<Bark>, std::ptrdiff_t>::value,
                  "Bark's difference_type should be ptrdiff_t!");

    int main() {}

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